1. Field of the Invention
The present invention relates to a polarization measuring apparatus for detecting the polarization of input signal light by measurement of the Stokes parameter or the like.
2. Description of the Related Art
In optical communication systems, as one means for increasing the transmission capacity, it is considered that a communication speed per one channel is increased. However, if the bit rate of a signal light reaches a range exceeding 10 Gbps (giga bit/second) or 40 Gbps, the pulse width of the signal light becomes several tens of ps (picosecond). Therefore, it becomes difficult to distinguish between a ‘0’ level and a ‘1’ level of the respective bits due to the waveform distortion caused by various factors. Since such waveform distortion becomes a factor in determining the main specifications such as system length, then when designing a system, various measures, such as arranging parts for compensating for the waveform distortion, are taken.
As a factor causing the waveform distortion of the signal light, there is polarization mode dispersion (PMD). This PMD is the dispersion which arises as a result of an occurrence of differential group delay (DGD) between two orthogonal polarization modes, due to, for example, the deformation of a core of an optical fiber to be used as an optical transmission path into an elliptic shape, a lateral pressure, a partial temperature change and the like. For example, in the case where an optical fiber is laid in a place, which is subjected to vibration or the like, along the side of a railroad, a change in the PMD is extremely fast, and a speed of the change is said to be approximately several KHz.
PMD compensators (to be referred to as PMDC hereunder) for compensating for the abovementioned PMD have been recently developed by various companies. A well-known PMDC configuration is basically a loop back system where the waveform distortion of a signal light is monitored and a compensation amount of the PMD is controlled corresponding to the monitoring result. However, according to such a loop back system, it is difficult to directly and quantitatively monitor a state of the waveform distortion and a generated dispersion amount. As substitute means, typically, there is a method for monitoring a degree of polarization (DOP). Moreover, examples of measuring the bit error rate (BER), or measuring the electrical spectrum hole burning are also known.
The DOP can be measured using a polarization measuring apparatus (polarimeter). As a conventional polarizabon measuring apparatus, there is known, for example, an apparatus for measuring four Stokes parameters representing polarization (for example, Japanese Unexamined Patent Publication No. 6-18332, Japanese Unexamined Patent Publication No. 9-72827, Japanese National Publication No. 2001-520754, and Japanese National Publication No. 2003-508772).
FIG. 7 shows a configuration of a basic optical system of the conventional polarization measuring apparatus as mentioned above. In this optical system, firstly an input signal light is branched into four at 25% each, by an optical coupler (CPL) 1. Then, a first branched light passes through a quarter wave plate (QWP) 2 and a polarizer (POL) 31 letting through only a polarization component which is inclined by 45° with respect to a preset reference plane, and is input to a light receiving element (PD) 41. A second branched light passes through a polarizer (POL) 32 letting through only a polarization component which is inclined by 45° with respect to the above reference plane, and is input to a light receiving element (PD) 42. A third branched light passes through a polarizer (POL) 33 letting through only a polarization component which is parallel (or perpendicular) with respect to the above reference plane, and is input to a light receiving element (PD) 43. A fourth branched light is directly input to a light receiving element (PD) 44.
If the electric signals which are photoelectrically converted by the respective light receiving elements 41, 42, 43 and 44, to be output, are DQ, D45, D0, and DT, then the four Stokes parameters S0, S1, S2 and S3 are represented by the relationship shown in the following equation (1).S0=DT S1=2·D0−DT S2=2·D45−DT S3=2·DQ−DT  (1)
Here, S0 represents the intensity of the input signal light, S1 represents a horizontal linear polarization component (0°), S2 represents a linear polarization component which is inclined by 45°, and S3 represents a right-handed rotatory circular polarizabon component. By using the abovementioned Stokes parameters S0 to S3, the DOP to be measured is represented in accordance with the relationship of the following equation (2).
                    DOP        =                                                            S                1                2                            +                              S                2                2                            +                              S                3                2                                                          S            0                                              (        2        )            
However, in the abovementioned conventional polarization measuring apparatus, there are following problems.
(a) Enlargement of the Apparatus Size
In the conventional polarization measuring apparatus, as shown in the optical system in FIG. 7, a large number of optical elements, such as, the optical coupler 1, the quarter wave plate 2, the polarizers 31 to 33, and the light receiving elements 41 to 44 must be arranged in required positions, and hence there is a tendency for the size of the whole apparatus to become large.
(b) Deterioration of Measurement Accuracy Due to Reflected Lights Generated in the Optical Elements
Generally, a part of an incident light is reflected at a light incident plane and the like of an optical element, a refractive index of which is changed. In order to suppress the generation of this reflected light, an anti-reflection film is normally formed on the light incident plane of the optical element. However, it is difficult to completely prevent the generation of the reflected light by the ant-reflection film. In the optical system shown in FIG. 7, there is a possibility that reflected lights are generated at the respective light incident planes of the optical coupler 1, the quarter wave plate 2, the polarizers 31 to 33, and the light receiving elements 41 to 44, and also, there are many places which can be reflection surfaces. In the case where some of these reflection surfaces are in a parallel or nearly parallel state with respect to a light emission plane of a former stage optical element, then for example as shown in FIG. 8, the multi-reflection of light occurs and an interference system is formed. Therefore, the power of the signal light detected by the light receiving element is varied with time, and a transmission characteristic has the wavelength dependence, resulting in the deterioration of measurement accuracy. Moreover, there is also a possibility that a part of the light reflected at the light incident and emission planes of the respective optical elements becomes a stray light. In an optical system where parts such as light receiving elements are arranged adjacent to each other in order to miniaturize the apparatus, the above stray light is input to a light receiving element different to a light receiving element to which the stray light is to be input primarily, to cause light leakage (cross-talk), resulting in the deterioration of measurement accuracy.
(c) Deterioration of Measurement Accuracy Due to the Phase Shift between p/s Waves
In the conventional polarization measuring apparatus, the input signal light is branched into four by the optical coupler 1, in order to obtain the four Stokes parameters S0 to S3. In the case where one utilizing for example a dielectric multi-layer film is used as the optical coupler 1, it is known that the phase shift occurs between the p wave (p polarized light) and the s wave (s polarized light) of the branched light (specifically, the transmitted light) due to the optical coupler 1. Such phase shift between the p/s waves does not cause a problem in a function of branching the optical power, but does change the polarization of the signal light after passing through the optical coupler 1. Therefore, in the case where there is an optical element such as a polarizer on the latter stage of the optical coupler 1, the phase shift affects the power of the signal light passing through the polarizer or the like, which becomes a factor in the deterioration of measurement accuracy.
(d) Deterioration of Measurement Accuracy Due to Temperature Fluctuation
Since the conventional polarization measuring apparatus comprises a large number of optical elements as shown in FIG. 7, characteristics of the respective optical elements are changed with the temperature fluctuation, which causes the deterioration of measurement accuracy. Moreover, since the input signal light is branched into four, and then transmitted over the respective optical elements, a mounted area becomes large and there is thus the likelihood of influence of optical axis shift due to the temperature fluctuation.